Ferromagnetic contacts can induce an exchange field into a nanodevice, but can also be used as a source of spin-polarized electrons and detector. In the latter case, so called tunneling magneto-resistance (TMR) devices are obtained when at least two contacts are ferromagnets (FM), the injector and detector contact. We have studied TMR physics in CNT-based quantum dots (QDs) early on.1 We have also found a pronounced exchange field that can lead to a splitting of the Kondo resonance is strongly coupled devices.2 Since ferromagnetic contacts turned out to be delicate, mostly because of their proneness to oxidation with an atomic-scale sensitivity of its magnetic pattern,3 we have studied single QD Coulomb resonances in the presence of FMs carefully and demonstrated that spurious signals due to e.g. the magneto Coulomb effect may not be effective in well prepared devices.4 In our most recent work, we have moved away from ferromagnetic contacts and explore the effect of an inhomogeneous stray field of nanomagnets in close proximity to CNTs and SNWs.5 This research is motivated by a recent theoretical proposal that demonstrate that a periodically varying magnetic field can mimic spin-orbit interaction. This synthetic spin-orbit interaction is interesting, since a simple array of ferromagnetic finger gates can induce large Rashba fields allowing to realize non-trivial topological states.

(a) Schematic of a NW with ferromagnetic side gates (FSG1 and FSG2) that result in a local stray field. The magnetization of the FSGs can be inverted by an external field B. (b) An actual device made from an InAs NW and Py nanomagnets. (c-d) Different measured traces for the magnetoresistance (MR) depending on the state of the NW QD. (c) is typically for the single QD regime, while d-e) require coupled QDs. One of the switches in the TMR-like curves in (e) is due to a ground-state change from a triplet to a singlet when lowering B.
  1. S. Sahoo, T. Kontos, J. Furer, C. Hoffmann, M. Graber, A. Cottet and CS, Nat. Phys. 1 (2), 99-102 (2005).
  2. L. Hofstetter, A. Geresdi, M. Aagesen, J. Nygard, CS and S. Csonka, Phys. Rev. Lett. 104 (24), 246804 (2010).
  3. S. Zihlmann, P. Makk, C. A. F. Vaz and CS, 2D Mater. 3 (1), 011008 (2016).
  4. J. Samm, J. Gramich, A. Baumgartner, M. Weiss and CS, J. Appl. Phys. 115 (17), 174309 (2014)
  5. G. Fabian, P. Makk, M.H. Madsen, J. Nygård, C. Schönenberger, A. Baumgartner, Phys. Rev. B 94, 195415 (2016).

Funding: SNF

Relevant papers (keyword: K_Ferro):

  • A Double Quantum Dot Spin Valve
    A. Bordoloi, V. Zannier, L. Sorba, C. Schönenberger, and A. Baumgartner.
    Communications Physics  3, 135 (2020)
    [arXiv:1912.02136 ] [ Open Data ] [Abstract ]

    We introduce a new route for semiconductor spintronics based on individually spin polarized quantum dots (QDs), obtained using ferromagnetic split-gates (FSGs). As proof of principle we demonstrate a double QD spin valve consisting of two weakly coupled QDs formed in an InAs nanowire (NW), each with an independent FSG with two allowed magnetization directions. We use electrical tunneling magnetoresistance (TMR) measurements to identify the two parallel (p) and two anti-parallel (ap) FSG magnetization states, and find a ~ 7\% reduction of the zero (external) magnetic eld conductance in the ap state compared to the p state, corresponding to an on resonance single dot spin polarization of ~30\%. The TMR and QD spin polarization can be signifficantly improved by a small (40 mT) homogeneous external magnetic field, which results in a TMR thatcan be continuously gate-tuned between \pm 90\%. A simple resonant tunneling model quantitatively reproduces all our ndings, allowing us to extract an electrically tunable QD spin polarization between \pm 80\%. Our results demonstrate that QDs with FSGs can be used as highly effcient and tunable in situ spin injectors and detectors in semiconductor devices, suitable, for example, for spin correlation experiments in a Cooper pair splitter, or to demonstrate equal spin Andreev reflection in Majorana devices.

  • Large spin relaxation anisotropy and valley-Zeeman spin-orbit coupling in WSe2/Gr/hBN heterostructures
    S. Zihlmann, A. W. Cummings, J. H. Garcia, M. Kedves, K. Watanabe, T. Taniguchi, C. Schönenberger, and P. Makk.
    Phys. Rev. B  97, 75434 (2018)
    [arXiv:1712.05678 ] [Abstract ]

    Large spin-orbital proximity effects have been predicted in graphene interfaced with a transition metal dichalcogenide layer. Whereas clear evidence for an enhanced spin-orbit coupling has been found at large carrier densities, the type of spin-orbit coupling and its relaxation mechanism remained unknown. We show for the first time an increased spin-orbit coupling close to the charge neutrality point in graphene, where topological states are expected to appear. Single layer graphene encapsulated between the transition metal dichalcogenide WSe2 and hBN is found to exhibit exceptional quality with mobilities as high as 100 000 cm2/Vs. At the same time clear weak anti-localization indicates strong spin-orbit coupling and a large spin relaxation anisotropy due to the presence of a dominating symmetric spin-orbit coupling is found. Doping dependent measurements show that the spin relaxation of the in-plane spins is largely dominated by a valley-Zeeman spin-orbit coupling and that the intrinsic spin-orbit coupling plays a minor role in spin relaxation. The strong spin-valley coupling opens new possibilities in exploring spin and valley degree of freedoms in graphene with the realization of new concepts in spin manipulation.

  • Spin transport in two-layer-CVD-hBN/graphene/hBN heterostructures
    M. Gurram, S. Omar, S. Zihlmann, P. Makk, Q. C. Li, Y. F. Zhang, C. Schönenberger, and B. J. van Wees.
    Phys. Rev. B  97, 45411 (2018)
    [arXiv:1712.00815 ] [Abstract ]

    We study room-temperature spin transport in graphene devices encapsulated between a layer-by-layer-stacked two-layer-thick chemical vapor deposition (CVD) grown hexagonal boron nitride (hBN) tunnel barrier, and a few-layer-thick exfoliated-hBN substrate. We find mobilities and spin-relaxation times comparable to that of SiO2 substrate-based graphene devices, and we obtain a similar order of magnitude of spin relaxation rates for both the Elliott-Yafet and D’Yakonov-Perel’ mechanisms. The behavior of ferromagnet/two-layer-CVDhBN/ graphene/hBN contacts ranges from transparent to tunneling due to inhomogeneities in the CVD-hBN barriers. Surprisingly, we find both positive and negative spin polarizations for high-resistance two-layer-CVDhBN barrier contacts with respect to the low-resistance contacts. Furthermore, we find that the differential spininjection polarization of the high-resistance contacts can be modulated by dc bias from −0.3 to +0.3 V with no change in its sign, while its magnitude increases at higher negative bias. These features point to the distinctive spin-injection nature of the two-layer-CVD-hBN compared to the bilayer-exfoliated-hBN tunnel barriers.

  • Fork stamping of pristine carbon nanotubes onto ferromagnetic contacts for spin-valve devices
    J. Gramich, A. Baumgartner, M. Muoth, C. Hierold, and C. Schönenberger.
    Physica status solidi (b)  252(11), 2496-2502 (2015)
    [arXiv:1504.05693 ] [Abstract ]

    We present a fabrication scheme called ‘fork stamping’ optimized for the dry transfer of individual pristine carbon nanotubes (CNTs) onto ferromagnetic contact electrodes fabricated by standard lithography. We demonstrate the detailed recipes for a residue-free device fabrication and in-situ current annealing on suspended CNT spin-valve devices with ferromagnetic Permalloy (Py) contacts and report preliminary transport characterization and magnetoresistance experiments at cryogenic temperatures. This scheme can directly be used to implement more complex device structures, including multiple gates or superconducting contacts.

  • Entanglement Detection with Non-Ideal Ferromagnetic Detectors
    P. Rozek, P. Busz, W. Klobus, D. Tomaszewski, A. Grudka, A. Baumgartner, C. Schönenberger, and J. Martinek.
    Acta Physica Polonica A  127(2), 493 (2015)
    [Abstract ]

    Entangled states are essential in basics quantum communication protocols and quantum cryptography. Ferromagnetic contacts can work as a spin detector, giving possibility of converting information about electron spin to the electric charge, and therefore, detection of entangled states with the electric current measurements is possible. Method of conrming entanglement with non-ideal detectors is presented, the impact of decoherence and noise on states and quality of entanglement is discussed. Entanglement witness (EW) operator method is compared with the CHSH inequalities approach. Required spin polarization for the EW is lower than for the CHSH inequalities. System with asymmetric spin polarizations of detectors was analyzed, including the CHSH inequalities and the EW method.